you're mixing up ramsay and roose. roose was the one who stabbed robb at the red wedding and ramsay is the one torturing theon. he's roose's bastard, hence the name ramsay snow. so the letter was written, as signed by ramsay (roose's bastard)
He isnt mixing them up. He is saying (wondering) that the letter that Theons dad received was not from Roose but from Ramsey pretending to be Roose.